The present invention relates to antifriction bearings, and more particularly to antifriction bearings which are suitable for use in an oil contaminated with extraneous matter.
Antifriction bearings for use in oils contaminated with extraneous matter include those already known and comprising bearing rings and rolling members which are made from a steel material containing 0.5 to 1.2 wt. % of carbon and 0.7 to 3.0 wt. % of chromium and which are subjected to a carburizing treatment. The surface layer portion of the raceway surface of each of the bearing rings and the surface layer portion of the rolling surface of each of the rolling members are 1.5 to 3.0 wt. % in carbon content and at least 63 in Rockwell hardness C and include a carburized layer which contains carbides precipitating in the form of fine globular particles, up to 10 xcexcm in diameter, in an amount of 15 to 80% in terms of area ratio. The matrix of the carburized layer has a carbon content of 0.6 to 0.7 wt. % see JP-A No. 41934/1995).
However, it has been found that the conventional antifriction bearing described fails to have a satisfactory service life since antifriction bearings are used under conditions of ever-increasing severity in recent years.
An object of the present invention is to overcome the above problem and to provide an antifriction bearing having a longer life than in the prior art.
The present invention provides an antifriction bearing comprising bearing rings and rolling members which are made from a steel material containing 0.15 to 0.3 wt. % of carbon by being subjected to heat treatments including carburizing, a surface layer of raceway surface of each of the bearing rings and a surface layer of rolling surface of each of the rolling members being 1.0 to 1.5 wt. % in carbon content, 64 to 66 in Rockwell hardness C, 150 to 2000 MPa in compressive residual stress, up to 3 xcexcm in maximum carbide particle size and 10 to 25% in carbide area ratio, at a depth of 0 to 50 xcexcm as measured from the outermost surface of the surface layer, the surface layers being 0.75 to 1.3 wt. % in carbon content, 150 to 1000 MPa in compressive residual stress, 25 to 45% in residual austenite content, up to 1 xcexcm in maximum carbide particle size and up to 15% in carbide area ratio, at a depth of 50 to a/5 xcexcm (wherein a is the effective case depth in xcexcm) as measured similarly.
The steel material to be used is, for example, one containing 0.15 to 0.3 wt. % of carbon, 1.2 to 1.6 wt. % of chromium, 0.35 to 0.55 wt. % of silicon and 0.35 to 0.65 wt. % of manganese, the balance being iron and inevitable impurities.
The antifriction bearing of the present invention has a prolonged life in clean oils and also in oils contaminated with extraneous matter.
The steel material serving as the blank material for the antifriction bearing of the present invention should be limited to 0.15 to 0.3 wt. % in carbon content for the following reason. The process of carburizing followed by quenching serves to produce a difference in carbon content between the carburized surface layer and the blank material to thereby assure the antifriction bearing of required compressive residual stress due to transformation during quenching. If the carbon content of the steel blank material is less than 0.15 wt. %, the internal hardness required of the antifriction bearing is not available, whereas if the content is in excess of 0.3 wt. %, the above-mentioned compressive residual stress due to transformation during carburizing and quenching will diminish. Accordingly the carbon content to be determined of the steel blank material should be within the range of 0.15 to 0.3 wt. %.
In the antifriction bearing of the present invention, the surface layer of raceway surface of the bearing ring and the surface layer of rolling surface of the rolling member should be 1.0 to 1.5 wt. % in carbon content, 64 to 66 in Rockwell hardness C, 150 to 2000 MPa in compressive residual stress, up to 3 xcexcm in maximum carbide particle size and 10 to 25% in carbide area ratio at a depth of 0 to 50 xcexcm as measured from the outermost surface the surface layer, in order to give the antifriction bearing the required rolling fatigue resistance, wear resistance and impression resistance. Stated more specifically, if the carbon content is less than 1.0 wt. %, the compressive residual stress is below 150 MPa or the maximum carbide particle size is in excess of 3 xcexcm, the required rolling fatigue resistance is not available, whereas even if the carbon content is n excess of 1.5 wt. % or the compressive residual tress exceeds 2000 MPa, the effect to improve the rolling fatigue resistance will level off, failing to produce increased resistance. Further when the Rockwell hardness C is less than 64, the required impression resistance will not be obtained, whereas if the hardness exceeds 66, the effect to improve the impression resistance will level off without increasing further. Further if the carbide area ratio is less than 10%, the required wear resistance is not available, whereas even if the ratio is in excess of 25%, the effect to improve the wear resistance will level off, failing to give higher resistance. Incidentally, the carbon content of the surface layer at the depth of 0 to 50 xcexcm as measured from the outermost surface thereof is the combined amount of the carbon contained in the carbide precipitates and the carbon contained in the matrix.
With the antifriction bearing of the present invention, the surface layer of raceway surface of the bearing ring and the surface layer of rolling surface of the rolling member should be 0.75 to 1.3 wt. % in carbon content, 150 to 1000 MPa in compressive residual stress, 25 to 45% in residual austenite content, up to 1 xcexcm in maximum carbide particle size and up to 15% in carbide area ratio at a depth of 50 to a/5 xcexcm (wherein a is the effective case depth in xcexcm) as measured from the outermost surface of the surface layer, in order to give an improved rolling fatigue life to the antifriction bearing. More specifically stated, if the carbon content, the compressive residual stress and the residual austenite content are less than the respective lower limit values, the required rolling fatigue life is not available, whereas if these values are in excess of the respective upper limits, the effect to improve the rolling fatigue life will level off, failing to give a further prolonged life. Especially, the residual austenite affords improved toughness when in an amount of 25 to 45%, exerting a favorable influence on the rolling fatigue life. More preferably, the residual austenite content is 25 to 35%. While nonmetallic inclusions frequently provide starting points of spalling due to rolling fatigue, coarse carbide particles can also be starting points of spalling, so that the carbide particles are limited in size and amount. Especially it is more preferable that the carbide area ratio be up to 7%. In the case where the surface layer of raceway surface of the bearing ring of the antifriction bearing and the surface layer of rolling surface of the rolling member thereof contain such carbide particles as mentioned above at a depth of 0 to 50 xcexcm as measured from the outermost surface of the surface layer, it is actually impossible to reduce the carbide content to zero at a depth of 50 to a/5 xcexcm (wherein a is the effective case depth in xcexcm) as measured similarly. However, it is desirable that the carbide content be zero if possible. Incidentally, the carbon content of the surface layer at the depth of 50 to a/5 xcexcm (wherein a is the effective case depth in xcexcm) as measured from the outermost surface thereof is the combined amount of the carbon contained in the carbide precipitates and the carbon contained in the matrix.